1. Field of Invention
The invention relates to a fixation apparatus that fixes using heat and pressure an unfixed toner image onto, for example, a recording sheet in an image forming apparatus using an electronic photography method, such as a copy machine, a printer or a facsimile. More particularly, the invention relates to an improvement of a fixation apparatus of a belt nip method with an endless belt arranged in contact with a fixing roll used in applying heat or pressure.
2. Description of Related Art
Applicant has proposed a fixation apparatus employing what is known in the art as the belt nip method, for example, in Japanese Patent Application No. 7-65629.
Such a fixation apparatus includes, for example, a heating roll having a heat source and being capable of rotation and an endless belt which is pressed onto said heating roll. The endless belt moves in conjunction with the heating roll. A pressing member arranged on the inside of the endless belt presses the endless belt against the heating roll, thereby forming a contact nip between the endless belt and the heating roll. The arrangement is such that an unfixed toner is fixed by heat and pressure onto a recording sheet by the recording sheet being passed through the contact nip region.
In this kind of fixation apparatus which uses a belt nip method, it is possible to secure a wide contact nip region with relatively low load in comparison, for example, with the roll pair method (a method with a heating roll and a pressure roll arranged in contact). By reducing the load, the core of the heating roll does not require the bending strength needed in the conventional model, and consequently can be made thinner than in the conventional model. Hence, it is possible to shorten the time required to raise the temperature of the heating roll from room temperature to a temperature where the fixation action is possible. In addition, because the pressing member is anchored in a non-rotating state, it is possible to reduce the heat absorption from the heating roll.
Further, concerning the endless belt, there are no, for example, suspension rolls. If the arrangement is such that there is substantially no contact outside the contact nip region, there is no excess radiation of heat, and heat absorption from the heating roll is reduced.
Through these measures, it is possible to reduce the time required to take the fixation apparatus from a waiting state to a state where the fixation action is possible, hereafter called the warmup time. By making this warmup time on the order of 15-30 seconds, it is possible to start heating of the fixation apparatus from room temperature from the time of reception of a signal to start image formation, thereby enabling image formation without a wait. That is, the fixation apparatus of the belt nip method conforms to so-called instant start.
However, in the above-described fixation apparatus which uses a belt nip method, technical problems arise. The endless belt tends to walk in one of the belt shaft directions because of differences in perimeter length on the two edges of the belt and differences in the load on the two edges of the contact nip region, or the degree of non-parallelness with the heating roll during belt installation.
The conventional method of avoiding this belt walk phenomenon (known in the art as belt walking) uses a mechanism for controlling the walking of the belt by displacing at least one of the suspension rolls, as in the fixation apparatus disclosed in Japanese Laid-Open Patent Publication No. 5-150679. However, this method requires an endless belt walk detection mechanism and the above-described roll displacement mechanism and the like, and hence is expensive.
In addition, from the perspective of reducing the radiation of heat from the endless belt, employing the above-described walk prevention mechanism is difficult when tension in the circumferential direction does not work on the endless belt.
Thus, a method of preventing walking by placing the belt edge in a guide member, for example, has been considered. This type of method which restricts the edge position of the endless belt except the contact nip region using a guide member is already known (for example, see Japanese Laid-Open Patent Publication No. 4-44075).
When this method is utilized, the belt receives a walking force from the contact nip region with the pressure roll, and movement occurs along the shaft direction. On the other hand, the edges of the belt receive a walk limiting force from the guide member when the belt collides with the guide member. At this time, the walking force and the walk limiting force are parallel. However, because these forces are not collinear, a moment acts on the belt. Technical problem arises that warping, twisting and buckling occur in the belt because of this moment. Further, with this kind of moment, a high rigidity is required in the belt so that warping, twisting and buckling do not occur in the belt.
Highly rigid belt materials, metal, and in particular, SUS and iron, can be found, but it is difficult to produce an endless belt having no seams. There are also belts which have been made seamless using welding, but the reliability of the weld spot is insufficient. In addition, electrocast belts made of nickel can be formed seamlessly, but these belts are not desirable from the standpoint of safety.
Alternatively, the strength is low in resin belts such as those made of polyimide and polytetrafluoroethylene. Consequently warping, twisting and buckling occur as described above. In order to compensate for this, it is possible to use a thick belt, but this is undesirable because of increased costs.